Wireless sensors have a great deal of potential use in numerous applications where a wired readout is difficult, for example, due to harsh operating conditions, rotating parts or cost and complexity of wiring. Wireless sensors are passive, battery-assisted semi-passive, or active containing a radio transmitter and a battery. The advantages of passive sensors are that their life-time or operation conditions are not limited by the battery and that they are inexpensive.
Mostly used passive wireless sensors include radio frequency identification (RFID), surface acoustic wave (SAW) RFID, electrical resonance circuit sensors, and harmonic and intermodulation sensors. RFID is mostly used for identification but will increasingly be used to realize sensors by adding a sensing element to the tag. The highest operation frequency and read-out distance of RFID is limited by the power rectifier that generates power for the IC and they are 5-10 m and a few GHz, respectively. An additional sensor element further increases the power consumption.
SAW RFID tags transform the electromagnetic energy to surface acoustic waves propagating on a piezoelectric substrate. The SAWs are then manipulated and transformed back to electromagnetic waves. The SAW tags lend themselves well as sensors as the propagation properties of SAWs are inherently sensitive to several measured quantities, such as temperature or strain, and no external sensor element is needed. The SAW sensors are reviewed in scientific literature. The highest operation frequency is typically limited to a few GHz by the line width of acoustical reflectors fabricated on the substrate. In addition, the necessity to use a piezoelectric material as the sensing element may limit the number of applications.
Inductively-coupled electrical resonance-circuit sensors are utilized for example to measure strain and moisture. These sensors consist of a simple electrical resonance circuit, whose resonance frequency is sensitive to the measured quantity, but they cannot be read across large distances as they require near-field coupling to the reader device.
Mixer sensors contain a mixing element, such as a diode, and they transmit the sensor data either at a harmonic or intermodulation frequency when illuminated by the reader device. Harmonic radar and tags were first proposed for traffic applications and later were used for tracking insects and avalanche victims. The intermodulation principle was first proposed for telemetry and later was used to implement wireless ferroelectric temperature sensor, proposed for a MEMS sensors based on mechanical mixing, and for automotive radars to detect pedestrians, cyclists and other road users who are in high risk of serious injury in a traffic accident. The advantage of the intermodulation principle over harmonic is the smaller frequency offset, which facilitates the circuit design and compliance with the frequency regulations. Generally, harmonic and intermodulation sensors can provide a very large frequency and read-out distance.